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The present invention generally relates to the preparation of liquid hydrocarbons from natural gas/methane, oxygen and/or steam. In particular, the present invention relates to an improved method for adjusting the hydrogen to carbon monoxide ratio in a synthesis gas product stream prior to addition into a hydrocarbon synthesis reactor.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons, alcohols and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis, or to alcohols through alcohol synthesis.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or catalytic partial oxidation (xe2x80x9cCPOXxe2x80x9d). Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Reaction (1).
CH4+H2O←xe2x86x92CO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. The steam reforming reaction is endothermic (the heat of reaction (1) is about 9 kcal/mol of methane), requiring the expenditure of large amounts of fuel to produce the necessary heat for the industrial scale process. Another drawback of steam reforming is that, if used as a Fischer-Tropsch feedstock, the 3:1 ratio of H2:CO products requires the removal of hydrogen to obtain the desired hydrogen to CO ratio of about 2.1 to about 2.5. Also, the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
The catalytic partial oxidation (xe2x80x9cCPOXxe2x80x9d) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial or direct oxidation of methane theoretically yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Reaction (2):
CH4+xc2xdO2←xe2x86x92CO+2H2xe2x80x83xe2x80x83(2)
The H2:CO ratio for this reaction is closer to that desired for the downstream conversion of syngas to chemicals such as methanol or other fuels than is the H2:CO ratio from steam reforming. In addition, the CPOX reaction is exothermic (xe2x88x928.5 kcal/mol-methane), in contrast to the endothermic steam reforming reaction. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes, i.e., short contact time reactors, which is impossible in a conventional steam reforming process. All of these factors lower the cost for the conversion of methane or natural gas and make the CPOX reaction much more attractive for commercial use.
Although there is a theoretical H2:CO ratio of 2:1 in the CPOX product stream at 100% conversion, in reality, the ratio ranges from about 1.6 to about 2.1 at about 70-99% conversion. Thus, CPOX reactions typically also require an adjustment of the H2:CO ratio prior to use as a Fischer-Tropsch feedstock. Because the steam reforming reaction is already more costly, in terms of energy consumption, it is logical to focus research efforts on ways to improve the H2/CO ratio of the CPOX reactions and/or efficient and cost effective ways to adjust the H2/CO ratio.
Despite research efforts to date, there is still a need for an improved method for the conversion of hydrocarbon gas to liquid hydrocarbons that includes a cost effective controllable process for adjusting the hydrogen to CO ratio of a synthesis gas product stream or Fischer-Tropsch feedstock stream.
The present invention allows for the adjustment of hydrogen concentration in a syngas product stream or Fischer-Tropsch feedstock stream. In particular, the invention provides an improved process for producing syngas comprising a secondary chemical reaction, preferably a water gas shift reaction, that allows for the adjustment of the hydrogen concentration in the syngas product stream. The water gas shift reaction is shown in Reaction (3). Ultimately, the present invention comprises an improved process for converting hydrocarbon-containing gas to liquid hydrocarbons.
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(3)
In one preferred embodiment, the improved process comprises (a) reacting a hydrocarbon-containing gas, such as methane or natural gas with oxygen, air or some other oxygen source in a syngas reactor to produce syngas; (b) obtaining a slip stream of the syngas of step (a); (c) reacting the slip stream from step (b) in a secondary reactor to produce a hydrogen rich product stream; and (d) introducing the hydrogen rich product stream of step (c) into the syngas stream of step (a) downstream of the slip stream removal of step (b). The volume of the slip stream can be controlled and the hydrogen rich product stream can be added back to the primary syngas stream in a controlled fashion such that an optimum hydrogen to carbon monoxide ratio could be obtained in the final combined streams.
In another preferred embodiment, the present invention comprises using the improved syngas product stream described above as a Fischer-Tropsch feedstock to produce liquid hydrocarbons. The hydrogen and carbon monoxide can be easily and continually adjusted so as to maintain an optimum ratio for the Fischer-Tropsch process.
According to the present invention, the syngas product streams of the various embodiments have adjustable hydrogen to CO ratios. The ratios are adjusted using a secondary chemical reaction, preferably a water gas shift reaction. According to the present invention, the ratio can be adjusted from about 1.6 to about 10.
These and other embodiments, features and advantages of the present invention will become apparent with reference to the following detailed description and drawings.